Methods of forming glass molds for eyeglass lenses include employing mechanical grinding and polishing methods, mechanical grinding methods, and electrical processing methods such as electrical discharge processing to produce a heat-resistant base mold, bringing this base mold into contact with a glass blank softened by heating to transfer the surface shape of the base mold, employing a grinding program for each surface shape to be obtained, and forming a base mold having a corresponding surface shape.
In recent years, the demand has increased for multifocal eyeglass lenses being made thinner and lighter by incorporation of axially symmetric, aspherical lens design. The hot sag molding method has been proposed (see Japanese Unexamined Patent Publication (KOKAI) Heisei Nos. 6-130333 and 4-275930, which are expressly incorporated herein by reference in their entirety) as a method for forming molds to produce eyeglass lenses having such complex shapes.
In the hot sag forming method, a glass material is placed on a mold, and softened by being heated to a temperature greater than or equal to its softening point, causing it to tightly contact with the mold. The shape of the mold is thus transferred to the upper surface of the glass material, yielding a formed article of desired surface shape. The glass material can be heated in a batch-type heating furnace or continuous heating furnace, but to achieve production efficiency, continuous heating furnaces are widely employed.
As the object being heated is being conveyed within a continuous heating furnace, it is possible to continuously conduct a series of processes within the furnace in the form of a temperature-rising step, an elevated temperature-maintaining step, a temperature-lowering step and the like by controlling the temperature within the furnace so as to impart a prescribed temperature distribution in the conveyance direction. However, in a continuous heating furnace, the amount of change in various parts of the surface of the object being heated tends to be nonuniform due to the presence of the temperature distribution in the conveyance direction, as stated above. For example, when a glass material is formed by the hot sag forming method in a continuous heating furnace having a temperature distribution such that the temperature rises from the inlet to the outlet, the glass material heats up rapidly and the amount of deformation increases greater at the front part. When the amount of deformation varies with the position of the glass material in this manner, the timing at which the lower surface of the glass material contacts the forming surface of the mold varies greatly with position, producing astigmatism that is unnecessary in eyeglass correction and sometimes compromising the sensation created by wearing the eyeglass because the deviation from the design value is asymmetric.
In contrast, Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-306390, which is expressly incorporated herein by reference in its entirety, proposes increasing the uniformity of heating by rotating the object being heated within the furnace in the course of sintering, metallizing, joining by brazing, and the like a ceramic product in a continuous heating furnace.
It is effective for the uniformity of heating to rotate the object being heated as described in Japanese Unexamined Patent Publication (KOKAI) Showa No. 63-306390. However, unanticipated astigmatisms are sometimes produced in the course of forming articles with complex shapes, such as casting molds for eyeglass lenses, by the hot sag forming method when achieving uniform heat distribution through simple rotation. In particular, unanticipated astigmatisms due to asymmetry are sometimes produced in the course of forming articles with free curved surface shapes lacking center symmetry, such as casting molds for progressive dioptric power lenses, by the hot sag forming method when achieving uniform heat distribution through simple rotation.